Apparatus for increasing the contrast in liquid immersion developing of electrostatic image



United States Patent [72] Inventor Arvhd R. Sakllltar Dork Forest, 111.[21] Appl. No. 625,405 [22] Filed March 23, 1967 [45] Patented Dec. 15,1970 [73] Assignee TheShervvln-Willlams Company Cleveland, Ohio acorporation of Ohio [54] APPARATUS FOR INCREASING THE CONTRAST 1N LIQUIDIMMERSION DEVELOPING OF ELECTROSTATIC IMAGE 5 Ch na, 2 Dram l ke.

[52] US. 118/637; 1 1 17/37 [51 Int. B$b /02 [50] Field Search 118/637;117/175. 37; 96/1.3; 355/ [56] References Cled UNITED STATES PATENTS3,068,l 12/1962 Gundlach 117/37 3,284,224 11/1966 Lehmann ll7/l7.5

5/1966 Ostensen Primary Examiner-Peter Feldman Attorneys-Thomas .1. McDowell and Richard G. Smith ABSTRACT: This invention comprises anapparatus for improving the deposition of particles upon a photographiccarrier having an electrostatic latent image on one surface, comprisingthe steps of passing the carrier into a tank of nonconducting liquiddeveloper having electrostatically attractable colloidal particlestherein, and between two electrodes submerged in the liquid developer,these electrodes being spaced from each other and being wide enough toembrace the entire width of the latent image on the carrier between thesuperim posed areas of the electrodes, while an alternating voltage of001-20, preferably 0.01-10 volts, is applied to these electrodes at afrequency of cycles to 1000 kilocycles per second.

PATENTEDBEBISISYG 3547.076

' INVENTOR ARV/ND R. .SAKL IKAR BYwManW ATTORNEY APPARATUS FORINCREASING THE CONTRAST IN LIQUID IMMERSION DEVELOPING OF ELECTROSTATICIMAGE This invention relates to improvement in apparatus for developingphotoconductive layers carrying latent electric images by a developmentmethod which is known as Liquid Immersion Development." In particular itrelates to improvements whereby the image can be developed to give amore intense deposition of particles resulting in. a higher degree ofdefinition and permanence.

In electrophotography, a plate or paper is used which has a layer ofphotoconductive material, such as light-sensitive zinc oxide which willaccept and hold an electrostatic charge. This plate or paper is exposedin a manner to vary the charge thereon, according to the pattern ordesign to be reproduced and the plate or paper is subsequently developedby immersing it in an insulating liquid containing the colloidalparticles, such that when it is immersed in the liquid, colloidalparticles will be deposited in conformity with the charge on the varioussections of the paper, thus producing a desired image.

The pigment or particle material'is preferably carried in a highresistance liquid which will not destroy the charge on the paper, atleast during the development period. It is extremely important to obtainuniform coverage of the paper to insure a buildup of the chargedparticles in exact proportion to the image left by exposure of thecharged plate. The colloidal particles must be uniformly and rapidlydeposited in a density varying in proportion to the charge held by thevarious parts of the image, so as to accurately reproduce the originalimage as translated by the variations in charge applied to the paper.One of the problems in existing processes is to obtain deposition ofparticles in sufficient density in the relatively short time the imageis being developed.

In the present method it is generally difficult to obtain uniformcoverage of the paper with a deposition of the colloidal particles inexact proportion to the image left by the exposure of the charged plate.In other words, it is difficult to effect deposition of the colloidalparticles in sufficient quantities and only in those areas where thecolloidal particles have the correct polarity in relation to the chargeon the paper.

In accordance with the present invention, it has surprisingly been foundthat the application of an alternating current on both sides of thepaper, as more fullydescribed hereinafter, effects a marked improvementin the intensity of colloidal particle deposition in accordance with thevariation in the charge on the paper. This effect of the'alternatingcurrent, or more truly alternating voltage, is entirely unexpected.-Since the process is based on the electrostatic charge on the paper'andthe attraction of the charged paper for particles having more or lessstatic charges, it might be expected that a static voltage or directcurrent application would be more appropriate. Nevertheless, it is nowfound that the application of an alternating current or alternatingvoltage is much more effective in increasing the intensity of particledeposition.

As described more fully hereinafter, the alternating current oralternating voltage is applied to two electrodes, one placed on eachside of the coated paper as it is being passed through the developingliquid. In cases where the paper is passed below a guide or roller, theguide or roller can be made to serve as one electrode and the bottom ofthe developing liquid tray or reservoir can be used as the otherelectrode, provided they are spaced an appropriate distance from eachother. Preferably the guide or roller and the bottom of the tray aremade of metal so as to be conductive. However, if either or both ofthese are made of plastic or other nonconductive materials, they can belined or coated with a thin metal foil or layer so as to serve aselectrodes forthe purpose of this invention. As an illustration of themethod'use'd in the practice of this invention, paper or other basematerial is coated with zinc oxide and an alkyd resin suspended in adrier and thinner so that after application of the solution the papercan be dried. Appropriate zinc oxide compositions for this purpose areknown in the art.

After drying, the paper is charged electrostatically by subjecting it toa corona discharge. For'example, this is effected by placing the paperon a flat metal plate electrode beneath a metal point connected to thenegative terminal of a 10 to 20 kilovolt DC source, the flat metal platebeing connected to the positive electrode. Then the paper is subjectedto the corona discharge for the an appropriate period such as 10seconds, while the point is held 2 inches above the paper. In place ofthe point, a wire or group of wires, .or a sharp steel knife edge may besubstituted. In such cases, the distance may be reduced between the wireor knife and the paper and the voltage reduced to give a correspondingelectrical field in terms of volts per cm.

Alternatively a charge may be applied on the paper by positioning itbetween condenser plates. After such a charge is applied over thesurface of the paper, the paper is exposed by subjecting it to theaction of light in a camera or to any other method of causing thedesired image to be produced on the electrostatically charged surface ofthe paper, the image being caused by the action of the light, etc: onthe charged surface.

Subsequent development of the image is effected by simply immersing thepaper in liquid developer which comprises a liquid insulating mediumcarrying colloidal particles. For example, the liquid can be acyclohexane having a very high resistance and a low dielectric constant.In the liquid is dispersed carbon black or other suitable pigment and acontrol agent modifier to give to the pigment the required electricalcharge. Thus, for imparting a positive charge to the pigment, an alkydresin control agent and fixer may be included in the liquid. Preferablythe liquid immersion developer (L.I.D.) may contain carbon black,crystal steep asphalt and a metallic salt of rosin as described incopending U.S. application, Ser. No. 485,549, filed Sept. 7, 1965 nowU.S. Pat. No. 3,399,140. lfa negative charge is desired, linseed oil canbe used as the con trol agent and fixer.

The liquid is applied by submerging the paper beneath the surface of theliquid or by pouring it on or spreading it on with a roller or otherapplicator. In most instances, the paper is fed under a guide submergedin the liquid so that as the paper passes under the guide, it is alsosubmerged in the liquid and passes through for the desired submersiontime.

In the drawings, FIG. I shows a cross-sectional view of a developingtank unit for electrostatic printing adapted for the practice of thisinvention by conversion of different portions of the equipment toelectrodes between which the electrostatic paper is passed during thedevelopment process.

FIG. 2 shows a portion of a bottom of a developer reservoir and aportion of an applicator roller between which the sensitized orelectrostatic paper passes during the developing process, the bottom ofthe tank serving as one electrode and the applicator roller serving asthe other electrode.

The toner trays in most commercial LID electrostatic photocopiers areplastic. These can either be replaced by metal trays or the plastictrays can be'lined with aluminum foil to serve as one electrode.Moreover, in most commercial apparatus of this type, a piece of highlyconductive metal such as copper or steel is submerged in the toner-bathand held above the paper as an aid in clarifying and intensifying theimage. This piece of metal is generally called an intensifier or bias.

However, it is generally specified that this piece is to be free ofelectrical charge and is generally insulated from grounding. Thisintensifier or bias can be used as the second electrode. However it isdesirable that the two electrodes cover the entire width of the paperand preferably a substantial part of the length of the paper as it ispassed into the bath between these two electrodes.

The effectiveness of the alternating current field is dependent somewhaton the voltage and the frequency of the alternating current. Moreover,the range of operability depends somewhat on the material used on thepaper, the toner itself, the electrodes and their spacing. In general,however, the variable frequency alternating current voltage isadvantageously in the range of 0.01 volts to 20 volts, preferably0.01-10 volts. and the frequency range is advantageously in the range of20 cycles per second to 1000 kilocycles, preferably 40- 120 cycles.

Optimum results are obtained at about volts with a frequency of 90- l 20cycles. With 10 volts and lOO cycles per second, the black density hasbeen found to be 57 percent higher than is obtained without anyelectrical field applied. Except for the electrical field describedherein and the elec trodes for the application thereof. the apparatus,the materials used and the method of preparation are generally similarto those previously used. However, the toner system must benonconductive and the process for developing the images must beelectrophoretic. Furthermore, the bias is designed and mounted in such away that the paper does not come into actual contact with it.

The apparatus of this invention and the manner of its use are bestdescribed by reference to the drawings. FIG. 1 shows apparatus suitablefor the immersion of carrier 1 having an electrostatic latent image inthe upper surface thereof. This carrier, illustratively zinc oxidecoated paper as described herein is fed by the ingress rolls 2 and 2into liquid developer 3 which is contained in reservoir 4. Reservoirbottom 4 is made of metal so as to serve as one of the electrodes towhich an alternating voltage is applied.

A metal bias 5, preferably copper, has a portion submerged in the liquiddeveloper and serves also as a guide for carrier 1 in its path throughthe liquid developer and to egress rolls or squeezing rollers 6 and 7.These rolls remove all excess liquid from the carrier 1. Roller 6 ismade up ofa resilient material such as rubber, while roller 7 ispreferably steel or a similar hard material so that sufficient pressurecan be applied to carrier 8 to remove the excess liquid. Wiper 8 iscontinuously pressed against the squeezing roll 7 to wipe it clean ofany pigment particles left by contact of the roll with carrier 1.

The liquid developer from reservoir 4 flows slowly into the main tank 9through the small opening 10 in the bottom of reservoir 4. The rate offlow from reservoir 4 through opening 10 into main tank 9 is much slowerthan the rate of pumping the liquid developer from the tank 9 intoreservoir 4. The desired level in reservoir 4 is maintained by anoverflow outlet, not shown, so that liquid will overflow from reservoir4 into main tank 9.

While the reservoir 4 is made of metal or at least lines with metal, themain tank 9 can be made of plastic material or other nonconductivematerial. Reservoir bottom 4 is fastened to a supporting portion of maintank 9 by means of bolt 11. The liquid developer in reservoir 4 iscontinually refreshed by replacement with liquid from the main tank 9from which liquid is withdrawn through pump system inlet 12 by pump 13and fed to the reservoir 4 through pipe 14' and header 14.

Metal member or electrode 5 is held in position by support means notshown and is connected to an alternating current electrical source byconductor 15 with reservoir bottom 4 being connected to the other poleof the alternating current source by conductor 16.

As previously indicated, when an alternating voltage of l0 volts andcycles per second is applied to the two electrodes, the resultant blackdensity has been found to be approximately 57 percent higher than isobtained when the electrical field is not applied.

FIG. 2 shows a cross-sectional view of a portion of apparatus adaptedfor the purpose of this invention by converting a metallic applicatorroller 5 to serve as an electrode and the metallic bottom of a reservoir4 containing liquid developer 3. Carrier 1 is shown in the liquiddeveloper passing along the bottom of the reservoir between theelectrode formed by applicator roller 5' and reservoir bottom 4 and outof contact with applicator roller 5'.

I claim:

1. Apparatus for developing by immersion in a liquid developer aphotographic carrier having an electrostatic latent image in one surfacethereof comprising:

a. a reservoir for a liquid developer ha'ving electrostaticallyattractable pigment particles therein;

b. a carrier'ingress means adjacent to one end of said reservoir;

. a carrier egress means adjacent to the opposite end of said reservoir,said carrier ingress and egress means being adapted to guide saidcarrier through said liquid developer;

d. a first electrode positioned below the path of said carrier in itspassage through said liquid developer;

e. a second electrode positioned above the path of said car rier in itspassage through said liquid developer, said second electrode beingpositioned so as to be immersed in said liquid developer contained insaid tank, and appropriately positioned so that said carrier may bepassed between said first electrode and said second electrode and sothat said carrier is out of contact with said second electrode, theareas of said electrodes superimposed and spaced from each other beingof sufficient width to cover that width of carrier which is desired tobe developed and also covering at least a substantial portion of thelength of said carrier; and

f. a means for supplying an alternating voltage to said electrodes.

2. The apparatus of claim 1 in which second electrode is also aconductive metal bias.

3. The apparatus of claim 1 in which said second electrode is also aconductive metal guide means.

4. The apparatus of claim 1 in which said second electrode is also aconductive metal roller.

5. The apparatus of claim 1 in which said first electrode comprises atleast a portion of the bottom of said reservoir and said portion of saidbottom is made ofa conductive metal.

